1. Field of the Invention
The present invention relates to a polystyrene foamed sheet suitable for forming. More particularly, it relates to a polystyrene foamed sheet stock to be used for making cups and other deeply drawn containers or for making trays and other thin-walled formings, said polystyrene foamed sheet stock is superior in formability and suitable for mass production.
2. Description of the Prior Art
One of the known methods for producing polystyrene foamed products employs an extruder. According to this method, a polystyrene resin composition and a blowing agent are extruded together by an extruder into a foamed sheet (sheet stock). The sheet is then heated above its softening point and formed into a desired shape using a mold. This forming method, however, suffers from several disadvantages. That is to say, the resulting formed part is poor in dimensional accuracy and stability (reproduction of mold) on account of the secondary foaming that takes place when the sheet is heated for forming. Such a formed part is not suitable for a lid that needs snug fitting. When heated for forming, the sheet cannot be drawn enough to be made into a cup and other deeply drawn containers. The forming cycle (time from sheet supply to demolding) is long. The resulting formed part is unduly bulky and takes a large volume when stacked.
The conventional method for continuously producing cuplike containers having a draw ratio greater than 1 from a polystyrene foamed sheet has another disadvantage. That is to say, it is necessary to juxtapose the molds with sufficient intervals, as shown in FIG. 3, because the elongation of the softened sheet is limited. (If sufficient intervals are not provided, the resulting formed part will break or decrease in strength.) The use of such a mold leads to poor productivity. It is desirable to produce as many formed parts as possible from a given sheet stock having a certain size. This is particularly important for the volume production of cuplike containers for instant foods (such as noodles).
Efforts have been made to improve the reproduction of mold and the formability by incorporating a filler and a rubber component into the foamed sheet. Such improvements, however, often caused breakage of sheet stock in the molding cycle and led to brittle products having low compression strength.
This invention has been completed in order to overcome the above-mentioned disadvantages. Accordingly, it is an object of this invention to provide a polystyrene foamed sheet suitable for forming. It is another object of this invention to provide a polystyrene foamed sheet stock which can be formed efficiently into deeply drawn containers.
The present inventors previously found that it is possible to form continuously trays and the like without performing aging of sheet, if the quantity of residual gas of blowing agent is controlled below 0.3 mole/kg at the time of sheet extrusion and foaming. (See U.S. Pat. No. 4,426,065.) The present invention is based on this finding, but a further improvement was made as the result of the studies carried out from a different point of view.